1. Field of the Invention
The present invention relates to defect reduction of non-polar and semi-polar III-Nitrides with sidewall lateral epitaxial overgrowth (SLEO).
2. Description of the Related Art
Gallium nitride (GaN) and its ternary and quaternary compounds are prime candidates for fabrication of visible and ultraviolet high-power and high-performance optoelectronic devices and electronic devices. These devices are typically grown epitaxially as thin films by growth techniques including molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), or hydride vapor phase epitaxy (HVPE). The selection of substrates is critical for determining the III-Nitride growth direction. Some of the most widely used substrates for nitride growth include SiC, Al2O3, and LiAlO2. Various crystallographic orientations of these substrates are commercially available which cause a-plane, m-plane, or c-plane growth of GaN.
FIGS. 1(a) and 1(b) are schematics of crystallographic directions and planes of interest in hexagonal GaN. Specifically, these schematics show the different crystallographic growth directions and also the planes of interest in the hexagonal wurtzite GaN structure, wherein FIG. 1(a) shows the crystallographic directions a1, a2, a3, c, <10-10> and <11-20>, and FIG. 1(b) shows planes a (11-20), m (10-10) and r (10-12). The fill patterns of FIG. 1(b) are intended to illustrate the planes of interest, but do not represent the materials of the structure.
It is relatively easy to grow c-plane GaN due to its large growth window (pressure, temperature and precursor flows) and its stability. Therefore, nearly all GaN-based devices are grown along the polar c-axis. However, as a result of c-plane growth, each material layer suffers from separation of electrons and holes to opposite faces of the layers. Furthermore, strain at the interfaces between adjacent layers gives rise to piezoelectric polarization, causing further charge separation. FIGS. 2(a) and 2(b), which are schematics of band bending and electron hole separation as a result of polarization, show this effect, wherein FIG. 2(a) is a graph of energy (eV) vs. depth (nm) and represents a c-plane quantum well, while FIG. 2(b) is a graph of energy (eV) vs. depth (nm) and represents a non-polar quantum well.
Such polarization effects decrease the likelihood of electrons and holes recombining, causing the device to perform poorly. One possible approach for eliminating piezoelectric polarization effects in GaN optoelectronic devices is to grow the devices on non-polar planes such as a-{11-20} and m-{1-100} plane. Such planes contain equal numbers of Ga and N atoms and are charge-neutral.
Another reason why GaN materials perform poorly is the presence of defects due to lack of a lattice matched substrate. Bulk crystals of GaN are not widely available so it is not possible to simply cut a crystal to present a surface for subsequent device regrowth. All GaN films are initially grown heteroepitaxially, i.e., on foreign substrates that have a lattice mismatch to GaN.
There is an ever-increasing effort to reduce the dislocation density in GaN films in order to improve device performance. The two predominant types of extended defects of concern are threading dislocations and stacking faults. The primary means of achieving reduced dislocation and stacking fault densities in polar c-plane GaN films is the use of a variety of lateral overgrowth techniques, including single step and double step lateral epitaxial overgrowth (LEO, ELO, or ELOG), selective area epitaxy, cantilever and pendeo-epitaxy. The essence of these processes is to block (by means of a mask) or discourage dislocations from propagating perpendicular to the film surface by favoring lateral growth over vertical growth. These dislocation-reduction techniques have been extensively developed for c-plane GaN growth by HVPE and MOCVD.
The present invention is the first-ever successful execution of sidewall lateral epitaxial overgrowth (SLEO) of non-polar a-plane and m-plane GaN by any growth technique. Prior to the invention described herein, SLEO of a-plane and/or m-plane GaN had not been demonstrated.